Sea urchins, macroalgae and coral reef decline: a functional evaluation of an intact reef system, Ningaloo, Western Australia

Despite being one of the world's most intact coral reef ecosystems, Ningaloo Reef in Western Australia exhibits unexpectedly high abundances of macroalgae and sea urchins. This challenges conventional indicators of reef decline, as extensive macroalgal beds and dense sea urchin populations were observed on the reef slope, where grazing rates by fish were also exceptionally high—completely scraping the substrate every 43 to 59 days. The study, led by Johansson et al., used functional evaluations to assess bio-indicators of reef health, revealing that echinoids significantly contributed to bioerosion on reef slopes, matching the impact of the most abundant parrotfish species. This finding underscores the need for caution when interpreting macroalgal and sea urchin densities as sole metrics for evaluating reef condition.

Johansson, C., Bellwood, D., & Depczynski, M. (2010). Sea urchins, macroalgae and coral reef decline: a functional evaluation of an intact reef system, Ningaloo, Western Australia. Marine Ecology Progress Series, 414, 65–74. https://doi.org/10.3354/meps08730